Filament life and bearing life are two important factors that limit the life of an x-ray tube. Therefore, x-ray tube bearing life is critical to high performance x-ray tube operation. In an x-ray tube, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is conducted and radiated to other components within the vacuum vessel of the x-ray tube. As a result of these high temperatures caused by this thermal energy, the x-ray tube components are subjected to high thermal stresses that are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. Further, to accelerate the electrons, a typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel along a focal spot alignment path. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed along the focal spot alignment path to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector or film, and an image is formed of the internal anatomy therein. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks, or to inspect the contents of luggage at airports.
Since the production of x-rays in a medical diagnostic x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700° C., while the temperature in the other parts of the anode may range up to about 1800° C. Additionally, the components of the x-ray tube must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450° C. for a relatively long duration. The thermal energy generated during tube operation is typically transferred from the anode, and other components, to the vacuum vessel.
The high operating temperature of an x-ray tube is problematic for a number of reasons. The exposure of the components of the x-ray tube to cyclic, high temperatures can decrease the life and reliability of the components. In particular, the anode assembly is typically rotatably supported by a bearing assembly. This bearing assembly is very sensitive to high heat loads. Overheating the bearing assembly can lead to increased friction, increased noise, and to the ultimate failure of the bearing assembly.
The choice of materials for such bearing assemblies in x-ray tubes is currently somewhat restrained because the rolling elements and the bearing itself must be electrically conducting in order to ensure electrical conductivity through the bearing to the cathode and anode assemblies. As a result, tool steel coated with a solid lubricant, such as lead, or more often, silver, is generally used in such bearings. However, the coating process is an expensive process, and the solid silver lubricant is thermally sensitive, generally requiring that bearings using such materials be operated at temperatures below 450° C. Additionally, traditional silver coated metal rolling elements generally deform substantially during operation, thereby resulting in noise and early failure of the bearing when operated at high temperatures for prolonged periods of time. Furthermore, silver also tends to react with the bearing steel if it becomes too hot, causing grain boundary cracking and premature failure of the bearing. Therefore, it would be desirable to be able to use materials in such bearings that did not have all the drawbacks of the current materials.
Electrically conducting bearings, made entirely of ceramic, would be ideal for such applications. However, no suitable electrically conducting ceramic bearings presently exist for such purposes. Thus, there is a need for such suitable electrically conducting ceramic bearings. Such ceramic bearings would ideally comprise Ti3SiC2 (also called TSC) instead of the steels or ceramics that are now commonly used (i.e., T5, T15, Rex20, SiN, Al2O3, SiC, zirconia, etc.). Such ceramic bearings would ideally not require the use of additional lubricant as many existing bearings do, thereby eliminating the expensive coating process that is currently required in some bearing assemblies. Additionally, such ceramic bearings would ideally allow faster rotation speeds and higher operating temperatures to be sustained than currently possible with existing bearings. Furthermore, the ceramic rolling elements in such ceramic bearings would ideally exhibit less deformation during operation than current silver coated steel rolling elements, and would ideally allow for quieter and smoother operation than currently possible. Moreover, such ceramic bearings would ideally lead to longer bearing life than currently possible with existing bearings. Many other needs will also be met by this invention, as will become more apparent throughout the remainder of the disclosure that follows.